Heretofore, various kinds of optical memories have been proposed which allow recorded information to be read out by irradiating a light beam on an information recording medium and detecting light reflected from it; examples of such optical memories include a ROM type memory which records information in the form of phase pits, a write once optical memory which records information by forming holes in a recording film by applying a light beam, a phase change optical memory which records information by causing a crystalline phase change in the recording film by applying a light beam, and a magneto optical memory which records information by applying a light beam and a magnetic field and thereby changing the direction of magnetization in the recording layer.
In these optical memories, signal readout resolution has been determined almost always by the wavelength λ of the reading light and the numerical aperture (N.A.) of the objective lens, and the minimum detectable pit spacing has been limited to about λ/(2·N.A.). Since it is not easy to reduce the wavelength of the reading light or increase the numerical aperture of the objective lens, attempts have been made to increase the information recording density by improving the recording media or readout methods. Among others, for magneto optical recording media, various strategies have been proposed to increase the information recording density. For example, Japanese Unexamined Patent Publication No. Hei 6-290496 discloses a technique that achieves a readout resolution exceeding the detection limit determined by the wavelength and the numerical aperture of the objective lens, by sequentially displacing the magnetic domain wall coming into the spot of the reading light beam and detecting the displacement of the magnetic domain wall. In this technique, if the readout layer, a first magnetic layer where the magnetic domain wall coming into the spot of the reading light beam is displaced, is formed in such a manner that adjacent information tracks are magnetically isolated from each other, a particularly good readout signal can be obtained.
However, to provide stable magnetic isolation between adjacent information tracks in the magnetic layer, the prior art has required that laser annealing be applied between the information tracks. Laser annealing, however, has had the problem that it is a time consuming process, and there has been a need for a method that can provide magnetic isolation between adjacent information tracks in a simpler way.
On the other hand, if a method is employed that achieves isolation between adjacent information tracks by using an optical disk-substrate having a land-groove configuration with a deep groove structure, the laser annealing step can be eliminated, but this method in turn has had the problems that the land-groove patterns with a deep groove structure are difficult to transfer when producing a substrate by injection molding, and that groove noise on the optical disk substrate increases because of the deep groove structure.
The present invention has been devised to resolve the above problems associated with the prior art, and it is an object of the invention to provide a magneto optical recording medium, and a method for reading the same, that can achieve high speed readout of signals whose recorded mark size is smaller than the diffraction limit of the information recording/reading light spot, and that can greatly improve the recording density and transfer speed, while eliminating the need for laser annealing and reducing the groove noise on the substrate.
The invention also provides a magneto optical recording medium having a structure such that the width of a groove area, which is a rewritable area, is 0.8 μm or less and height difference between the land and groove areas is within a range of 20 nm to 80 nm, and that the surface roughness of the land area is 1.5 nm or more; with this structure, the magneto optical recording medium achieves excellent signal characteristics that can suppress a ghost signal being copied from the rear of the light spot when a DWDD system is used for readout. The invention is also directed to the provision of a readout method for such a magneto optical recording medium.